Joe Ross (Campbell Scott), constantly scribbling equations in his notebook, has invented something of immense value to the company he works for. Referred to only as "the process" or "the formula," it promises to bring in millions. The company flies Joe down from New York to a tropical paradise to pitch the process to a group of executives, and while there Joe meets and befriends wealthy playboy Jimmy Dell (The Out-of-Towners' Steve Martin, wonderfully sinister), also from New York. As Joe is about to leave for home, Jimmy asks Joe to deliver a small package to his sister in New York. Joe agrees, to the consternation of Susan (Rebecca Pidgeon), the new secretary from Joe's office, along on the trip as a go-fer. What do they know about Jimmy?, she asks Joe. Did he actually come from the seaplane that landed near the dock where Joe met him, or did Joe just assume that because Jimmy's boat came from that direction? Did Joe see what he thought he saw, or did he just think he saw it?

Susan warns Joe: "You never know who anybody is, with the exception of me. I am what I look like... Anybody could be anybody." Joe starts to wonder about Jimmy, but he once again looks without seeing when it comes to Susan -- she so obviously is not the annoying and obsequious secretary she's pretending to be, but only the audience is catching on to that at this early stage -- or are we?

Back in New York, Joe starts to suspect that the company is going to screw him out of his fair compensation for the process. His boss is giving him a doubletalk runaround. Susan seems to be trying to snoop around Joe's highly secure private office. When Joe shares his concerns with Jimmy, his new friend suggests speaking to a lawyer. By now, we've no idea whom to believe in, except straight-arrow Joe. Even Mamet is not to be trusted -- his script has Jimmy saying that "good people, bad people, they generally look like what they are," but no one here fits into the standard movie stereotypes, and there are visual cues everywhere that we suspect are clues to what's going on, but maddeningly they can only make sense in retrospect.

Like The Sting and The Game, The Spanish Prisoner is of that delightful brand of movie that you can watch for the first time only once -- half the fun is thinking you've finally figured out precisely what's going on only to find your expectations continually shifting. Unlike the creators of those other films, though, Mamet seems intent on reminding us that all of it is fake. The strangely stilted performances from Scott and Pidgeon puzzled me at first, until I realized that Mamet wants us to remember that this is only a movie, that his actors are only pretending, that there's a whole other level of "watching but not seeing" to which moviegoers usually willingly subscribe but that the director doesn't want us to indulge in here.

The film's title refers to the "oldest con in the world," an FBI agent tells Joe. Mamet's con -- letting his audience know they're being duped and getting them to love it at the same time -- must be one of the newest.